This invention pertains to microlithography (projection-transfer of a pattern, as defined on a reticle or mask, to a substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as an energy beam. Yet more specifically, the invention pertains to charged-particle-beam (CPB) optical systems used in CPB microlithography apparatus, and to apertures used in such optical systems to form a hollow beam.
The progressive reduction of the sizes of circuit elements in microelectronic devices has led to the development of microlithography apparatus that use an energy beam other than ultraviolet light, so as to achieve finer resolution than obtainable using optical microlithography (i.e., microlithography performed using light). One promising approach has centered on the use of a charged particle beam (e.g., electron beam) as a microlithographic energy beam. Because the rectilinearity (and hence the resolution) of an electron beam tends to be better than of a light beam, microlithography apparatus using an electron beam have the potential of accurately exposing a pattern having smaller pattern elements than is possible using optical microlithography.
Charged-particle-beam (CPB) microlithography apparatus at their current state of development tend to exhibit low throughput (number of wafers or substrates that can be processed microlithographically per unit time). One way in which to increase throughput is to increase the beam current of the charged particle beam. However, increasing the beam current causes an accompanying increase in the particle density within the beam, which tends increasingly to aggravate the xe2x80x9cCoulomb effect.xe2x80x9d The Coulomb effect arises from electrostatic (Coulombic) repulsion between particles of like charge in the beam. The Coulomb effect causes, inter alia, beam blur, which substantially degrades the achievable pattern-transfer resolution obtained with CPB microlithography.
U.S. Pat. No. 5,834,783 discloses an exemplary technology for reducing the Coulomb effect in electron-beam microlithography. Specifically, the electron beam is made hollow (i.e., is configured to have a ring-shaped or annular transverse sectional profile) by passing the beam through a hollow-beam aperture. A hollow beam greatly reduces Coulombic repulsion between the electrons in the beam and, as a result, reduces the Coulomb effect. A typical hollow-beam aperture comprises an electron-absorbing plate defining a substantially ring-shaped (annular) through-hole and a circular center portion. The annular through-hole typically has multiple struts extending radially across it that serve to support the center portion. The center portion desirably absorbs electrons incident on it as other electrons pass directly through the annular through-hole. The hollow-beam aperture normally is situated on an optical axis at a location in the electron-optical system where electrons emitted by an upstream source converge (this location is termed a xe2x80x9ccrossoverxe2x80x9d).
The diameter of the beam at the crossover of a CPB-microlithography apparatus typically is approximately 100 xcexcm. Hence, the size of a hollow-beam aperture placed at the crossover must be very small. Specifically, the diameter of the circular center portion of the hollow-beam aperture should be about 60 xcexcm, and the radial width of the annular through-hole should be about 20 xcexcm. The hollow-beam aperture should be made of a material that effectively blocks (absorbs) charged particles (except for particles passing through the annular through-hole) and has a high melting point. Molybdenum is a particularly useful material for this purpose.
The conventional method for fabricating a hollow-beam aperture includes machining arc-shaped openings in a molybdenum sheet using an end mill or analogous cutting tool, as shown in FIG. 17. However, this method is incapable of cutting an annular through-hole having a very narrow radial width. A more suitable alternative method is electric-discharge machining (EDM), in which an electrode is situated very near the workpiece (molybdenum plate) where the aperture is to be formed. For example, the electrode is situated 20 xcexcm from the workpiece. High-voltage pulses are applied between the electrode and the workpiece to form an electrical arc across the gap between the electrode and the workpiece. The narrowest aperture that can be formed using EDM is equal to the diameter of the electrode plus 40 xcexcm (20-xcexcm gap on each side of the electrode). In other words, it is impossible to cut a 20-xcexcm wide annular opening using EDM. Therefore, no practical method currently exists for making a hollow-beam aperture having a desired width for placement at a beam crossover.
In view of the shortcomings of the prior art summarized above, an object of the invention is to provide hollow-beam apertures, for use in CPB microlithography, having very small radial widths, such as a radial width of approximately 20 xcexcm. Another object is to provide methods for manufacturing such hollow-beam apertures and to provide CPB microlithography apparatus comprising such hollow-beam apertures.
To such ends, and according to a first aspect of the invention, hollow-beam apertures are provided for incorporation and use in a charged-particle-beam (CPB) microlithography apparatus. A first embodiment of such an aperture comprises a first member, multiple second members, and a circular center member made of a CPB-absorbing material. The center member is supported relative to the first member by support bars extending radially from the first member to the center member. The second members are situated between the support bars and the first member, and are displaceable relative to the first member radially toward the center member. The second members each have a distal edge. The distal edges are configured to engage the support bars whenever the second members are displaced maximally toward the center member. The distal edges each define a cutout having a respective edge configured as an arc having a radius greater than the radius of the center member. Whenever the second members are displaced maximally toward the center member, an annular aperture is defined between the center member and the cutouts. The aperture is xe2x80x9csubstantially annular,xe2x80x9d i.e., annular except for the support bars extending across the aperture to the center member.
A respective spring can be situated relative to the first member and extending to each of the second members. Each spring is configured to urge the respective second member toward the center member. The springs desirably are contiguous with the first and second members, thereby connecting the respective second members to the first member.
The first member desirably defines angled edges in a region between the second members. In such a configuration, each second member defines angled edges that conform to and contact corresponding angled edges of the first member in a manner, whenever the second members are displaced maximally toward the center member, serving to maintain concentricity of the cutouts relative to the center portion. In such a configuration, the angled edges of the second members effectively xe2x80x9cfitxe2x80x9d into respective spaces defined by the angled edges of the first member. As the second members are urged closer to the center body, the engaging angled edges of the first and second members cause the second members to self-align relative to the first member and the center member, thereby assuring concentricity of the cutouts in the distal edges with the center member.
In a second embodiment of a hollow-beam aperture according to the invention, a first member defines a cutout having a radial dimension. A cylindrical beam-absorbing member is disposed relative to the first member. The beam-absorbing member has an axis and a radius, wherein the radius is smaller than the radial dimension of the cutout. Multiple support bars support the beam-absorbing member relative to the first member and concentrically with the cutout such that the axis of the beam-absorbing member is perpendicular to the plane of the first member, and the beam-absorbing member extends through the cutout. The first member, beam-absorbing member, and support bars can be machined from a single body of beam-absorbing material. This embodiment also includes a second member (desirably planar in configuration) that defines a circular cutout having a radius no larger than the radius of the cutout in the first planar member and larger than the radius of the beam-absorbing member. The second member desirably is configured for superposed attachment to the first member such that the cutout of the second member is coaxial with the beam-absorbing member, and the beam-absorbing member also extends through the second cutout. The radius of the cutout in the second member desirably is smaller than the radius of the cutout in the first member. Hence, when the first and second members are joined, they define a substantially annular aperture that is contiguous except at the support bars.
In this second embodiment, the second member can be manufactured after manufacturing the first member. Specifically, the beam-absorbing member of the first member can be used as an EDM electrode for forming the cutout in the second member. Using such a technique, the cutout in the second member readily can be configured concentrically with the beam-absorbing member. Furthermore, this technique allows the cutout in the second member to be formed having a radius that is only 20 xcexcm greater than the radius of the beam-absorbing member.
By providing such a narrow annular aperture, Coulomb effects in the CPB microlithography apparatus are controlled effectively, especially whenever the subject hollow-beam aperture is placed at a crossover.
In a third embodiment of a hollow-beam aperture according to the invention, a main body defines a circular opening having an axis. A beam-absorbing body is situated concentrically relative to the circular opening and is connected to the main body by at least one support bar contiguous with the main body and the beam-absorbing body. The beam-absorbing body has a radius that is smaller than the radius of the circular opening. The main body defines at least one void situated relative to the circular opening and the beam-absorbing body. The void is configured so as to cause the circular opening and beam-absorbing body to define a substantially annular aperture, when the hollow-beam aperture is viewed along the axis, extending through the main body and concentric with the beam-absorbing body.
To form the structure of this embodiment, machining of the main body is performed from multiple directions. If machining were performed only from one direction, according to conventional practice, then an annular aperture having a sufficiently narrow radial width (e.g., 20 xcexcm) could not be formed. By machining from multiple directions, the requisite narrow radial width can be obtained, even if the dimensions of the individual machining cuts are large.
As noted above, the resulting aperture that is formed is xe2x80x9csubstantially annular.xe2x80x9d Such an aperture can be contiguous except for support bars traversing it. Furthermore, whereas a substantially annular aperture generally is circular, the term xe2x80x9csubstantially annularxe2x80x9d also includes apertures that are, for example, elliptical (but not elliptical to such an extent that significant anisotropy would be a problem during use). The term also includes apertures not having a uniform radial width around the circumference of the aperture (but again, not to such an extent that significant anisotropy would result).
In a first example of this third embodiment, the at least one void comprises first and second voids, which can be situated on opposite sides of the axis. Each of the first and second voids can be cylindrical, with respective axes that are parallel to each other and perpendicular to the axis of the beam-absorbing body.
In another example, each of the first and second voids extends, at an angle relative to the axis, through the main body to the circular opening. In this second example, each of the first and second voids can be rectangular or trapezoidal in profile (in the latter configuration, the voids have tapered sides).
In this embodiment, the beam-absorbing body and the circular opening desirably are each defined by respective portions of the main body that are relatively thick in a beam-transmission direction and are separated from each other by a portion of the main body that is relatively thin in the beam-transmission direction. In such a configuration, the voids desirably are machined so as to remove some of the portion that is thin in the beam-transmission direction, leaving the support bar defined by remaining portions that are relatively thin in the beam-transmission direction. The beam-absorbing body desirably is rotationally symmetric (about the axis). The resulting substantially annular aperture has a radial width equal to the difference between the radius of the beam-absorbing body and the radius of the circular opening. The achievable radial width is very narrow, including as narrow as 20 xcexcm.
Further with respect to this embodiment, the machining steps can be performed in any order, with the same advantageous results obtained in any event.
The dimensional accuracy of the annular aperture (beam-transmitting portion) is determined by the accuracy of the diameter of the beam-absorbing body, and by the dimensional accuracy of the remaining portion that is relatively thin in the beam-transmission direction. Hence, the dimensional accuracy of the annular aperture is not largely dependent on the accuracy of machining operations performed to form the voids (by removing some of the xe2x80x9cportion that is relatively thin in the beam-transmission directionxe2x80x9d). As a result, the machining operations performed to form the voids can be relatively xe2x80x9croughxe2x80x9d and are thus easy to perform.
The circular opening can have a truncated conical profile as viewed along a direction perpendicular to the axis of the beam-absorbing body, wherein the circular opening extends into the main body from a first surface of the main body. In this configuration, the main body can define multiple (e.g., at least four) voids each having a respective axis that is parallel to the axis of the beam-absorbing body. The voids extend into the main body from a second surface of the main body opposite the first surface. The voids desirably are arranged such that their respective axes are equally spaced from one another about the axis of the beam-absorbing body in a rotationally symmetric manner (to eliminate anisotropy). Each of the voids intersects, within the main body, a respective portion of the circular opening in a partially overlapping manner. The partially overlapping portion of each void with the circular opening forms a respective portion of the substantially annular aperture, while intervening non-overlapping portions form the beam-absorbing body and support bars. The radial width of the substantially annular aperture can be determined by adjusting the diameters of the voids relative to the diameter of the circular opening. Hence, a substantially annular aperture is formed, having an extremely narrow radial width, by a simple process involving machining performed in only two directions.
Either of the machining steps (machining the circular opening and machining the multiple voids) may be performed before the other step.
Each of the voids can have a truncated conical profile as viewed along a direction perpendicular to the axis of the beam-absorbing body. The conical profiles provide tapered sides that scatter incident charged particles back toward the incident beam at an angle. This can reduce disturbances near the outside of the beam due to multi-path reflection.
In yet another example, the circular opening can be annular in profile with tapered outer sides and tapered inner sides, thereby forming a beam-absorbing body that is conical relative to the axis. Such a circular opening is formed on a first surface of the main body. The voids can be machined into a second surface of the main body opposite the first surface. Hence, machining is easy because it is performed from only two directions. Also, in this example, the edge of the annular aperture can be made sharp, wherein the edge is located in a plane perpendicular to the axis of the beam-absorbing body. Also because both the inner and outer edges of the annular aperture are determined by the same machining operation, they automatically reside on the same plane, resulting in highly accurate relative lateral positioning of beam-absorbing portions of the hollow-beam aperture. This eliminates anisotropy of the hollow beam formed by the aperture.
According to yet another embodiment, a hollow-beam aperture according to the invention comprises a charged-particle-stopping member defining a cutout extending along an axis through a thickness dimension of the charged-particle-stopping member. The aperture also comprises a support member that defines multiple openings. The support member is situated relative to the cutout in the charged-particle-stopping member so as to collectively define a substantially annular aperture coaxial with the axis. The hollow-beam aperture also can include a reinforcing member defining an opening that is coaxial with the axis. The charged-particle-stopping member, the support member, and the reinforcing member (if included) can be integrated as a unitary construct.
According to another aspect of the invention, methods are provided for manufacturing a hollow-beam aperture for use in a charged-particle-beam (CPB) microlithography apparatus. According to an example embodiment of a method according to the invention, a main body is provided made of a CPB-absorbing material. On a first surface of the main body, a circular opening is machined having an axis. On a second surface of the main body opposite the first surface, the main body is machined to define a beam-absorbing body concentrically relative to the circular opening. The beam-absorbing body has a radius smaller than the radius of the circular opening. The circular opening and the beam-absorbing body each have a relatively thick dimension in a beam-transmission direction and are separated from each other by a portion of the main body that is relatively thin in the beam-transmission direction. The main body is machined further to define at least one void situated relative to the circular opening and the beam-absorbing body. The void is configured so as to cause the circular opening and the beam-absorbing body to define a substantially annular aperture, when viewed along the axis, extending through the main body and concentric with the beam-absorbing body.
The circular opening can be machined to have a truncated conical profile as viewed along a direction perpendicular to the axis, and to extend into the main body from a first surface of the main body.
The step of machining the circular opening can comprise machining an annular groove in the first surface. In such an instance, the voids are machined into the second surface so as to intersect the annular groove. Hence, a substantially annular aperture is formed having tapered inner sides and tapered outer sides, as summarized earlier above.
According to another embodiment of a method according to the invention, a main body is provided that is made of a CPB-absorbing material. On a first surface of the main body, a circular opening is machined that has an axis and a radius. The circular opening defines a beam-absorbing body having a radius smaller than the radius of the circular opening. The circular opening and the beam-absorbing body each have a relatively thick dimension in a beam-transmission direction and are separated from each other by a portion of the main body that is relatively thin in the beam-transmission direction. The main body is machined to define at least one void situated relative to the circular opening and the beam-absorbing body. The void is configured so as to cause the circular opening and the beam-absorbing body to define a beam-transmitting aperture that is concentric with the beam-absorbing body and substantially annular in profile when viewed along the axis. The beam-transmitting aperture extends through the portion of the main body, between the beam-absorbing body and the circular opening, that is relatively thin in the beam-transmission direction.
According to yet another embodiment of a method according to the invention, a main body is provided having first and second main surfaces and a thickness dimension extending along an axis between the first and second main surfaces. Multiple second openings are defined, extending from the first main surface into the thickness dimension. The second openings are situated radially symmetrically relative to the axis. A third opening is defined extending from the second main surface into the thickness dimension. The third opening is situated coaxially with the axis and intersects the second openings in the thickness dimension so as to define a substantially annular aperture as viewed along the axis.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.